Air separation by rectification (at cryogenic temperatures) is well known. Typically, in such methods the air is separated in a double rectification column comprising a higher pressure rectification column, a lower pressure rectification column and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column. Such an arrangement enables an oxygen product to be withdrawn from a bottom region of the lower pressure rectification column. In addition, a nitrogen product is typically taken from the top of the lower pressure rectification column.
Normally, a relatively high yield or recovery of oxygen from the incoming air can be achieved by rectification of the air in a double rectification column. However, various demands may be placed on the separation such that the oxygen recovery will fall. Such demands include the production of liquid products in an amount in excess of 5% of the total oxygen production when refrigeration of the process is provided by turboexpansion of air into the lower pressure rectification column; a requirement for a liquid nitrogen product; and a requirement for a gaseous nitrogen product not only from the lower pressure rectification column but also from the higher pressure rectification column. The demands on the separation process are increased if an argon product is formed by withdrawing an oxygen stream containing argon from the lower pressure rectification column and separating argon from it in a side rectification column. Further, if an argon product is produced, co-production of a nitrogen product from the higher pressure rectification column or co-production of relatively large proportions of liquid products can have a drastic effect on the argon recovery.
U.S. Pat. No. 5,469,710 relates to an air separation method employing a double rectification column and a side column in which an argon product is produced, wherein oxygen-enriched liquid is taken from the bottom of the higher pressure rectification column, is passed through a throttling valve into a condenser in which argon is condensed, the oxygen-enriched liquid thereby being vaporised, and a stream of the resulting vapour is expanded with the performance of external work and introduced into the lower pressure rectification column. Such an arrangement is advantageous in that it is a useful way of providing additional refrigeration for the separation, thereby adding to the flexibility of the method in being able to provide liquid products without unacceptable product recoveries or unacceptable power consumption. The method is, however, limited by the fact that the argon condenser needs to be operated at a pressure less than 2 bar in order to provide the necessary temperature difference for the condensation of argon; therefore the amount of refrigeration that can be produced by expansion to the pressure of the lower pressure rectification column is strictly limited.
It is an aim of the present invention to provide a method and apparatus which enables oxygen recovery, and, if separated, argon recovery to be enhanced.